Corrected previews selection logic. Added method for deleting captures
to captures table model.
Added possibility to delete captures
Updated CaptureRectFrame to let it always know it's last on-screen
position so that capturing would not have to make it visible to
determine that position. Refactored capturing logic to ignore visibility
of CaptureRectFrame for sequence capturing, and only show/hide (with
delay) CaptureRectFrame for single-shot capturing (in case it's
visible).
Corrected single-capture showing/hiding delay workaround - only do delay
if actual showing/hiding of capture rect frame occurs (i.e. no delay if
it was hidden anyway).